PROJECT SUMMARY This project will explore adaptive immune mechanisms that control development of hepatocellular carcinoma (HCC), a leading cause of cancer-related deaths, and dictate its responsiveness to PD-1:PD-L1 checkpoint inhibitors. Our team, including Michael Karin, Ph.D. and Shabnam Shalapour, Ph.D. at UCSD School of Medicine and Hidekazu Tsukamoto, D.V.M., Ph.D. and Anthony El-Khoueiry, M.D. at USC Keck School of Medicine, represents an ideal blend of basic researchers, translational scientists and oncologists who are interested in HCC molecular pathogenesis and treatment, especially in HCC caused by non-alcoholic (NASH) and alcoholic (ASH) steatohepatitis. Although the US incidence of HCC and its associated mortality have nearly tripled in the past generation, insufficient effort has been made toward identification and development of innovative and effective HCC therapies. However, the ideal and timely confluence of basic preclinical research and applied clinical studies carried out by our team members has the potential to critically transform HCC treatment forever. Together we found that chronic liver inflammation results in suppression of HCC-protective immunosurveillance to support rapid malignant progression. This unique immunopathogenic mechanism renders HCC responsive to drugs that disrupt the PD-1:PD-L1 checkpoint, but even the impressive response seen thus far and the selection of patients who will benefit from this therapeutic approach can be further improved. Such improvements can only be achieved by a deeper understanding of the mechanisms through which PD-1:PD-L1 inhibitors act, the factors that determine their efficacy, and the causes of treatment failure. We will achieve these goals through integrated studies of clinical specimens collected by Dr. El-Khoueiry and sophisticated, faithful and robust mouse models of non-viral HCC developed by Drs. Tsukamoto, Shalapour, and Karin. The immune mechanisms that control NASH- and ASH-driven HCC development in these models are highly similar to those that operate in human patients. Using this integrated approach, we will pursue five specific aims: 1) determine whether serum IgA concentrations correlate with therapeutic response to PD-1 blockade in patients with non-viral HCC; 2) develop reliable mouse models of ASH-driven HCC; 3) compare the immunosuppressive mechanisms that contribute to development of NASH- and ASH-driven HCC and control their response to PD-L1 blockade; 4) determine whether excessive peritumoral fibrosis correlates with diminished response to PD-1 blockade in HCC patients; and 5) determine whether agents that inhibit or attenuate stellate cell activation potentiate the response to PD- 1/PD-L1 blockade in non-viral HCC. The successful completion of these studies will result in substantial improvements to HCC immunotherapy and will establish reliable procedures for identification of patients who are most likely to benefit from PD-1/PD-L1 targeting drugs, advances that will result in significant cost savings which may amount to hundreds of millions of dollars annually.